Localization of key mutations in the three-dimensional structure of Thermobacillus xylanilyticus xylanase (Tx-Xyn) . (A) Positions of Y3, Y6, S27 and Y111 residues. (B) Potential secondary binding site determinants (red), including S27 (blue), mutated in this study. (C) Space-filling model showing the spatial occupation of side chains at positions 111 and 121 in Tx-Xyn and in the mutants Y111H, Y111S and Y111T. All figures were prepared using PyMol software .